I can't quite figure out which of the following expressions is more correct:
- He is the devil's advocate.
- He is a devil's advocate.
- He is playing devil's advocate.
The combination of an article with the possessive is what confuses me. Exactly which word(s) does the article apply to?
The first form seems to suggest either that he is an advocate of The Devil -- namely, Satan himself -- or even worse, that he is The Advocate of The Devil. (Kill him with fire!)
The second form seems to suggest that he is an advocate of a devil (but not necessarily of The Devil, nor the only advocate out there.) This seems to fit better with the way this idiom is commonly used, but I haven't seen this idiom used very often with the indefinite article. It's usually used with the definite article.
The third form suggests that he is playing a role named "devil's advocate", with no article attached to it.
Similar examples: The King's speech, the Indian's prayer, the mother's room, etc.